Fish in the Bay – September 2020: Silverside Tsunami, spawning Anchovies & some apex predators.

Trawl map annotated with Anchovies and Silversides. Anchovy spawning locations are circled in pink.

Anchovies continue to spawn in September!  We detected Anchovies loaded with eggs or milt for the third month!  Our Anchovies seem to be spawning continuously through the warm season.  This should be no surprise since it is typical of other Anchovy populations off the coasts of North America and Europe.

The Anchovy Spawning Zone expands!  In July and August, we were unable to perform Anchovy ‘egg-checks’ at the 10 trawling stations downstream of the Railroad Bridge, so spawning status there was unknown.  In September, we checked the downstream stations and found more egg-bearing Anchovies.  Anchovies with eggs have now been observed at 15 of our 20 stations!!! 

Most likely, spawning Anchovies would be found in many other brackish marshes around San Francisco Bay … if we looked.


Anchovy and Silversides trend lines:  2020 is a very good Anchovy year.  The September Silverside Tsunami is off the chart!

Silverside Tsunami = Bad News!  Just when we thought we had this entire eco-system figured out.  The silversides exploded in September.  We found many hundreds of them in Ponds A21 and A19 and at  Stations Alv2, Art1, and Dmp1. 


Tide was flooding or at lower-high tide on both trawling days in September.

This Silverside Tsunami was not an artifact of trawling on a falling tide. 

  • You may recall that the explosion of 2,543 Topsmelt caught in Pond A21 in July was at least partly attributed to the low and ebbing spring tide that helped flush the small fish out of the marsh.
  • In September we were trawling into the flooding neap near lower-high tide. There was very little tidal velocity and no marsh flushing to disturb these massive schools of Mississippi Silversides.


Bay-side stations trawling results.


Upstream of Railroad Bridge.

Low Dissolved Oxygen (DO).  Dissolved Oxygen was below the official “Water Quality Objective” (WQO) of 5.0 mg/l nearly across the board.  (Granted, that WQO was established many years ago to apply only to the more oxygen-rich main stem of the Bay (LSB stations and further north)).

I now only highlight values below 4 in red in the tables. We learned long ago that marsh fishes tend to be very tolerant of low DO.  It now appears that Anchovies may continue to spawn down into the mid-2’s!!!


1. Silverside Tsunami

Thousands of Silversides and a few other fish from four stations.

Mississippi Silverside / Inland Silverside (Menidia berlina, or M. audens) are one of our most common invasive species.  They were first introduced as gnat and midge control at Clear Lake, CA in 1967.  They are now residents of fresh-to-brackish waters all over California. I do not know of any specific harm they are causing here, but they compete with native fishes, and that is not good.


Silverside closeup.

In our monthly trawls since 2014, Silverside numbers typically jump to several hundred in late summer through fall.  This month we caught 5,541.  We have never seen such huge numbers of Silversides in trawls going back to 2010.   Why now?


2. Spawning Anchovies – for the third month in a row.

Colorful adult Anchovies from station Alv1

Larger, darker, and more colorful Anchovies seem to be more prevalent at upstream stations. 

  • On Saturday at station Alv1, six out of seven measured 80 mm or longer; two measured 90 and 97 mm. At station Coy3 two were 80 and 105 mm long.  All the remaining 431 fish were less than 80 mm.
  • At stations upstream of the railroad bridge on Sunday, between a quarter to over one-half of Anchovies measured at all stations were over 80 mm in length.
  • I am beginning to think that Northern Anchovies may tend to become consistently darker and more colorful with age. (i.e: melanophore darkness and color intensity (not hue) from iridophores may give a rough indication of age.)


Alviso Slough and Pond A21 are now confirmed as a likely Anchovy spawning places.  From a small subsample, a total of eight egg-bearing females and 2 milting males were observed at stations Alv2 and Alv3.  In pond A21, ten egg-bearers and two milting males were discovered.

  • Interestingly, most egg-bearing females at all stations seem to be young and relatively unpigmented in July, August, and September.  Why do most egg-bearing females appear to be so young?


A few of the longer, darker, and more colorful Anchovies at Station Coy3.


Spawning Anchovies in Artesian Slough:  one egg-bearing female was verified at station Art2; five more were confirmed at Art3.


Anchovies and Silversides from Pond A19 in the tray.


Anchovies and Silversides from Pond A19 swimming in the tub.  This top-down view gives a better idea of Anchovy and Silverside appearance in the wild. 


Collage showing a range of Anchovy colors in Pond A19.


At all stations, egg-checks were performed only on small random subsamples of total Anchovies caught.   Seventeen egg-bearing Anchovies were confirmed in Pond A19!


Some of the larger dark and green Anchovies from Dmp2.


Younger, less pigmented Anchovies with eggs.  Eggs were confirmed in 23 Anchovies in Dump Slough!

Corbula Clams (Corbula amurensis) could threaten our Anchovy spawn by sucking up all the tiny food.  It seems that Northern Anchovies that used to be abundant in Suisun Bay became scarce shortly after the invasion of Corbula clams in 1987.

According to Kimmerer (2006)  “When C. amurensis invaded, the distribution of northern anchovy Engraulis mordax shifted toward higher salinity, reducing summer abundance by 94% in the low salinity region of the estuary. …  The shift in spatial distribution appears to have been a direct behavioral response to reduced food.”


Some of the 364 Corbula clams collected at station UCoy2 on 13 September.  Most of these had barnacles growing on them.  Could barnacles be yet another natural Corbula control?

Death to Corbula! (Corbula amurensis).  Our Corbula clam populations here are still small.  Ducks, sturgeon, and other natural controls seem to be keeping them in check in Lower South SF Bay.  But, we must never let our guard down!  Non-native Corbula must not be allowed to disrupt our happy Anchovies!  I continue to collect and remove all Corbula that I can.


3. Apex predators in Lower South San Francisco Bay.

A young adult Bat Ray.

Bat Ray (aka “Mud Marlin”).  We caught 14 Bat Rays in September.  It was a mix of adults and newborns.  This is pretty typical for summer through fall.  Both rays and sharks tend to be more common further north in San Francisco Bay and along the coast.  Lower South Bay marshes are their brackish-water upstream boundaries.


Leopard Shark, two views:  in the tub (top), and in the bucket ready for release.

Leopard Shark is our number one charismatic species.  These are found all over San Francisco Bay and along the California coast.  They eat clams, crabs, shrimp, seaweed, and small fish if they can catch them.  They grow up to five feet long and are completely harmless to humans as near as I can tell.  We never provoke them, but we handle them.  If they were inclined to bite, we would have been bitten many times by now.

We catch a few Leopard Sharks each year.  The Bay gets fresher this far south so it is on the fringe of Leopard Shark habitat.  When big rains give us a robust creek flushing, the Leopard sharks must flee north or die. 


Micah showing off one of the three baby Brown Smoothhounds caught in September.

Brown Smoothhound Shark.  This is our number two charismatic species (IMO).  We catch Smoothhounds less often. 


Brown Smoothhound newborns.

In September, we caught three Smoothhounds at LSB2!  It was a trio of newborns.  Smoothounds are live-bearers, so these literally could be triplets. 

Smoothhounds are small sharks.  They only grow to about 3 feet long at most.  (Larger sharks eat Smoothhounds, so technically these are not true apex predators.)  On both the Atlantic and Pacific coasts, they live in near proximity to the nearly identical Spiny Dogfish.  The identifying feature is presence or absence of a spine just in front of first and second dorsal fins.  Some people refer to Smoothhounds as just another type of “Dogfish.”


White Sturgeon:  1) in the tub, 2) Micah lifting the fish, 3) using the net as a sling for Sturgeon release.

White Sturgeon.  Once or twice per year, we catch one of these big ones.  I have said it all before, and I hate wasting words.  But, to find the ancient Triassic dinosaur fish with 250 million years of stable body-form-and-function ancestry is always a rush. It is very pleasing to find these big creatures growing on top of our estuarine food pyramid.   Is there anything this marsh cannot do?!

Note:  We think low DO may have throttled this sturgeon.  DO was less than 3 mg/l at Coy2!  Otherwise a big strong fish this size would most likely outswim our net.  Some big fish are like that; they just reduce activity when DO drops low.  All will be good as long as more oxygen sweeps in with the next tide, creek flush, or daily diatom bloom. 


Harbor Seals at Calaveras Point on 12 September.  These are our other apex predators.


4. A Big Mudsucker

Longjaw Mudsucker (Gillichthys mirabilis).  Speaking of low DO:  We caught this big guy swimming in the borrow ditch on the east side of Pond A19.  Many readers may know by now that Longjaw Mudsuckers can breathe out of water for at least 12 to 15 hours, or more, when they must.  In water, Mudsuckers simply gulp air when Dissolved Oxygen falls to near zero.  They can even squirm considerable distances over mudflats if caught by falling tide.  This is a very useful adaptation for living in stagnant brackish sloughs. 

According to McGourty, Hobbs, et al (2009), Mudsuckers are good sentinel species indicating presence of both high-quality marsh habitat and pollutants.  They tend to live their lives in one place (high site fidelity) and literally suck mud for small invertebrates. 


Longjaw Mudsucker in the photarium.

At 115 mm standard length, this male is just over the “Young of Year” (YoY) threshold.  He is growing a long jaw and big mouth that will allow him to battle for prime refuge and nest site up in the creeklets amongst pickleweed or tule roots.  With luck, he will continue to grow and reproduce over his 6-year lifespan. 

Normally, mudsuckers we catch are much smaller juveniles.  Big guys like this one tend to hold prime protected feeding areas and usually don’t venture out into exposed borrow ditches where the trawl can catch them.  Big Mudsuckers can be very good indicators of healthy marsh.


5. Oregon Mud Crab.

Oregon mud crab carapace is shaped like the state of Oregon  … coincidence?*

Oregon Mud Crab (aka Yellow Shore Crab, Hemigrapsus oregonensis).  After almost a year-long hiatus Oregon Mud Crabs seem to be coming back: we caught two in July, and one in August.  This month there were five.  The marine crabs are fewer but not gone: we caught three decorator crabs in September. 

I suspect that gulls, herons, and egrets keep our shore crab population under control.  Most of these birds will gladly make a meal of a mud crab, or a Mudsucker for that matter.

( * Micah actually pointed out this similarity of shapes between Oregon Mud Crab and the State of Oregon on the August weekend.)


Birds over the marsh in Pond A19.  Shore crabs and Mudsuckers: Take cover!


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